1. Field of the Invention
This invention relates in general to clamping circuits for clamping a signal at a predetermined reference level and, more particularly, to a clamping circuit for clamping a signal at a predetermined current level.
2. Description of Related Art
In television applications it is often desirable that the peak variations of the television signal, namely the tips of the sync signal, be clamped to a reference voltage potential called the "sync tip clamping level". This serves to prevent overmodulation or undermodulation of the television signal on a radio frequency carrier by placing the television signal excursions within a known range of voltage values.
One typical clamping circuit is disclosed in U.S. Pat. No. 4,914,324, issued to Goto, and is shown in FIG. 1A as clamping circuit 10. Clamping circuit 10 includes differential amplifiers 15 and 20 which are connected in a loop. An FET switch 25 completes the path which joins amplifiers 15 and 20. A holding capacitor 30 is coupled between one side of FET switch 25 and ground. An input voltage signal, V.sub.SIGNAL, is provided to the non-inverting input of amplifier 15 and an output signal, V.sub.OUT, is observed at the output of amplifier 15. A voltage reference signal V.sub.REF is provided to the inverting input of amplifier 20. Clamping circuit 10 acts to clamp the input voltage V.sub.SIGNAL to the V.sub.REF voltage reference signal to produce a clamped output voltage V.sub.OUT.
The operation of clamping circuit 10 is now briefly discussed with reference to the voltage vs. time graph, V.sub.OUT vs. TIME, shown in FIG. 1B. When FET switch 25 is turned on at time T1, a difference voltage between the V.sub.OUT output voltage and the V.sub.REF reference voltage is fed back to the inverting input of differential amplifier 15. The difference between V.sub.OUT output voltage and the V.sub.REF reference voltage is applied to the inverting input of differential amplifier 15 to correct the V.sub.OUT output voltage level. In this manner, the output voltage V.sub.OUT becomes clamped to a definite voltage which is equal to reference voltage V.sub.REF in this example. The output of differential amplifier 20 is held by a holding or smoothing capacitor 30 such that the clamping state is maintained.
FIG. 2 shows a video signal 50 both prior to clamping to a voltage reference V.sub.REF and after such clamping. More specifically, video signal 50 includes a video line 55 which is unclamped and a video line 60 which is clamped at V.sub.REF. As seen in FIG. 2, the sync tip 65 of video line 55 is below the V.sub.REF reference voltage, thus signifying that video line 55 is unclamped. In contrast, the sync tip 70 of video line 60 is at a value equal to the V.sub.REF reference voltage, thus signifying that video line 60 is clamped. Clamping of the sync tip to a reference level is often used in consumer television products such as video camcorders, whereas commercial television products often clamp the backporch region of the video signal to a voltage reference signal.
Most conventional clamping circuits deal with signals in terms of voltages and utilize capacitors and diodes to operate. For example, one such conventional clamping configuration is depicted in FIG. 3 as clamping circuit 100. Clamping circuit 100 performs DC restoration of an AC coupled signal, ie., a capacitively coupled signal. Clamping circuit 100 includes an input capacitor, C, for coupling an input signal to a diode D. Many practical clamping circuits employ an amplifier to form a superdiode structure. Clamping circuits often place a switch between the diode and ground to gate the clamping action.
While clamping circuits which operate on voltage signals are well known, in certain applications it is also desirable to have a clamping circuit which operates on signals in terms of current, as opposed to voltage.